This invention pertains to a hermetic rotary compressor for compressing refrigerant in refrigeration systems such as refrigerators, freezers, air conditioners and the like. In particular, this invention relates to an improved main bearing for rotatably supporting the crankshaft in a rotary hermetic compressor.
Prior art hermetic rotary compressors generally comprise a casing or housing surrounding the working parts of the compressor. The housing is hermetically sealed to prevent compressed gas from escaping and to prevent dust and other contaminants from entering the housing. Located within the housing are an electric motor for driving the compressor and a compressor pumping mechanism driven by the motor. The electric motor comprises a stator and a rotor. The stator is generally cylindrical in shape and the rotor is located inside the stator and drives the crankshaft. In general the stator is secured to the inside wall of the housing by shrink fitting. The crankshaft includes an eccentric portion which is rotatably received in the compression bore of a compressor cylinder. In many conventional prior art structures, the compressor cylinder is also secured to the housing by shrink fitting or welding. The cylinder assembly includes a roller which surrounds the crankshaft eccentric portion and is driven thereby inside the bore the cylinder assembly also includes one or more sliding vanes. The roller revolves around the bore of the cylinder as it is driven by the crankshaft and cooperates with the sliding vanes to compress refrigerant in the bore.
The dimensional tolerances necessary for proper operation of the compressor are extremely close and are generally on the order of ten thousandths of an inch. It is important that the tolerances be held very closely and to minimize gaps between working parts of the compressor to prevent leakage of compressed refrigerant and a resulting decrease in the efficiency of the compressor below acceptable levels.
The bore of the cylinder is concentric with the axis of the crankshaft and therefore needs to be aligned very precisely with the crankshaft, the crankshaft bearing and the rotor of the motor. Since in the prior art structures the stator and cylinder are attached to the housing and since the rotor is aligned with both the stator and the cylinder, the rotor must be well supported to maintain this alignment. It is crucial that the bearing is aligned with both the stator and the cylinder in order to prevent excessive gaps between the roller and sliding vanes.
One of the problems with the prior art compressors has been that the dimensional tolerances and the concentricity of the parts have been difficult to maintain during assembly of the compressor. Attachment of the cylinder and motor stator has generally been accomplished by shrink fitting and, therefore, in the prior art structures these parts have their entire circumferences in contact with the inside wall of the housing. Since the housing is relatively flexible, misalignment of the motor and cylinder can occur as the pressures within the pressurized housing fluctuate and the housing flexes. Misalignment in the motor causes air gap variations between the motor stator and rotor thereby adversely affecting performance of the motor. Furthermore, distortion can occur in the vane slot and cylinder during the shrink fitting or welding operation, thereby causing distortion and loss of clearance between the working parts of the compressor. In conventional designs, clearance must be added to compensate for this distortion, thereby increasing leakage and adversely affecting performance of the compressor. For this reason the prior art compressor cylinders have generally been of relatively heavy construction with a large axial dimension so that the process of securing the cylinders to the housing wall and the distortion forces generated thereby would not appreciably distort the cylinders and cause undesirable distortion.
In general the crankshaft is journalled in a bearing which in turn is attached to the compressor cylinder by means of threaded bolts, welding or the like. In one prior art structure the compressor bearing has been supported by a circular disc which was press fit in the housing of the compressor cylinder and welded to the housing at several points around its circumference. The housing was, therefore, in contact with the disc around its entire circumference. This structure is more expensive due to additional material and machining costs to maintain concentricity and close tolerances for press fitting to the housing.
A problem encountered with the above discussed prior art structures which use a thick cylinder with a large axial dimension has been that relatively long leakage paths exist in the compressor cylinder assembly, thereby decreasing the efficiency of the compressor. During operation of the compressor the various areas of the compressor contain refrigerant at various pressures. For instance, the bore of the compressor cylinder has both an inlet portion at suction pressure and a high pressure portion wherein the refrigerant is compressed. Furthermore, the compressor housing itself is at high pressure because compressed refrigerant is expelled from the cylinder bore directly into the housing. It is important to keep leakage of refrigerant from the high pressure areas to low pressure areas to a minimum, since such leaked refrigerant represents lost work and reduces the efficiency of the compressor. Therefore, it is important that the length of the borders dividing low and high pressure areas are made as small as possible. The height or axial dimension of the cylinder is a critical dimension affecting leakage since it is directly related to the border length dividing the high and low pressure areas in the compressor cylinder bore and around the sliding vane. For instance, the length of the seal between the sliding vane and the cylinder slot is a border dividing high and low pressures cylinder bore areas. By using a thin cylinder these critical border dimensions can be kept small and refrigerant leakage past the vane tip as well as other borders can be reduced as explained hereinabove. The problem with a thin cylinder is that welding of the cylinder to the housing causes distortion and leakage.
Another disadvantage of the heavy construction of the prior art compressor cylinders is that it adds to the weight of the compressor. Since hermetic compressors are used in household appliances compressors are preferably of lightweight construction.
One further disadvantage of the prior art structures is that the relatively large axial dimension of the compressor cylinder increases the surface area available for heat transfer to the refrigerant gas. Such heat transfer is undesirable and tends to decrease the efficiency of the compressor. It is therefore desirable that the heat transfer surface be minimized in order to optimize the efficiency of the compressor.
Yet another disadvantage of prior art rotary hermetic compressors is the cost of manufacture and assembly because of the relatively heavy construction of the compressor cylinders and the difficulty of assembling the structure to maintain close tolerances. Accordingly, it is desirable to be able to utilize a thin cylinder block.